Secondary radiation mitigation in electron-impact x-ray sources

Abstract

A transmission-type X-ray source is disclosed, comprising an electron source for providing an electron beam; a target configured to generate X-ray radiation by interaction with the electron beam; an X-ray transparent exit window; a plurality of electron beam limiting apertures, each configured to limit an angular distribution of the electron beam; a secondary radiation limiting aperture; a first deflector arranged to deflect the electron beam from the secondary radiation limiting aperture towards the target; a second optional deflector arranged to deflect the electron beam from a selected one of the electron beam limiting apertures towards the secondary radiation limiting aperture; and an aperture selector configured to make the selected one of the electron beam limiting apertures and the electron beam path coincide; wherein each of the electron beam limiting apertures and the secondary radiation limiting aperture are arranged so that there is no direct line of sight for secondary X-ray radiation from the electron beam limiting apertures to the exit window. The aperture selector may comprise a third deflector arranged to deflect the electron beam towards the selected one of the electron beam limiting apertures, or an actuator arranged to bring the selected one of the electron beam limiting apertures into the electron beam path.

Description

[0001] SECONDARY RADIATION MITIGATION IN ELECTRON-IMPACT X-RAY SOURCES

[0002] Technical field

[0003] The present disclosure relates to X-ray arrangements and X-ray sources. More particularly, the disclosure relates to mitigation of effects caused by secondary X-ray radiation, i.e. undesired X-ray radiation generated in a transmission-type X-ray source.

[0004] Background

[0005] X-ray radiation can be generated by directing an electron beam onto a target material. A typical prior art X-ray source thus comprises an electron source to generate the electron beam, and electron optics to direct and focus the electron beam onto the target material. Such electron optics may include magnetic alignment and / or stigmator coils, and electrostatic or magnetic focusing lenses and deflectors. The target material typically comprises a high-Z (e.g. atomic number Z > 20) material that generates X-ray radiation as bremsstrahlung and line emission upon electron impact. The target may comprise a target film, e.g. made from tungsten (W), deposited on a target substrate material, e.g. made from diamond. A typical thickness of the target film may be about 0.5 pm or several micrometers or several tens of micrometers, while a typical thickness of the target substrate may be about 100 pm or several hundreds of micrometers.

[0006] The generated X-ray radiation may be directed for use in, for example, X-ray imaging of a sample or an object.

[0007] Secondary radiation is a well-known problem within the art of X-ray sources. Secondary radiation is generally understood as any X-ray radiation other than the desired radiation and can originate, for example, from scattering of X-rays or from unintended electron beam interactions in the system. Hence, the source of secondary radiation may be scattering of the primary X-ray radiation, interaction of backscattered electrons with other source components, or interaction between the electron beam and some part of the source located upstream of the target. It is a common problem within the field that when this secondary radiation is emitted from the X-ray source it can contribute to measurement noise, artifacts, and / or ghost images in an X-ray detector arranged to detect the X-ray radiation generated by interaction between the electron beam and the target.

[0008] Improved techniques for addressing such secondary X-ray radiation are thus desired.

[0009] Summary

[0010] The present invention relates to a transmission-type X-ray source where X- ray radiation is created through interaction between an electron beam and a target.

[0011] A transmission-type X-ray source is an X-ray source in which generated X-ray radiation is extracted in the same general direction as the incoming electron beam. This means that the target must be able to transmit X-ray radiation. The target thus typically comprises a metal layer (i.e. a layer comprising at least one metallic element) deposited on a substrate, wherein the substrate is transparent to X-ray radiation. The substrate may, for example, be made of diamond and the metal layer may, for example, be made of tungsten. The target used in transmission-type X-ray sources is sometimes referred to as a transmission target. In some implementations, the target (or more particularly the substrate thereof) is also used as the exit window for X-ray radiation to exit the source, while other implementations may have a separate exit window with the target located upstream from the exit window.

[0012] To adjust a size of the X-ray emitting region (X-ray spot), the extent of the electron beam at target impact (electron beam spot) is adjusted accordingly. To avoid spherical aberrations of the electron beam, it is desirable to limit the angular distribution of the electrons entering an electro optical system (e.g. lenses) arranged to focus the electron beam onto the target. This is conventionally done by providing an aperture within the path of the electron beam. The aperture opening is selected to match the desired electron beam spot size.

[0013] In many cases, it is desirable to have different sizes of such aperture openings available. Within the context of electron beam control there are proposals on how to accomplish this where, in particular, mechanically exchanging or translating aperture plates or deflecting the electron beam to different aperture openings have been suggested. However, these solutions do not seem to be accepted within the field of X-ray sources. One issue with apertures within X-ray sources in general is that the electrons impacting material around an aperture opening cause unwanted secondary X-ray radiation. For example, X-ray radiation generated by interaction with such apertures may show up as distinct artifacts or create a secondary image of an object within the field of view having a different magnification than the image obtained from the primary X-ray source. This type of secondary X-ray radiation is especially problematic in transmission-type X-ray sources because any secondary radiation generated along the electron beam path towards the target will partly be directed towards the exit window. The secondary X- ray radiation may in this case typically result in a bright central part of the X-ray radiation emitted from the X-ray source. One known way to mitigate the effects of this bright central part is to place the detector off-axis so that X-ray radiation from this central part do not contribute to the X-ray radiation that is collected by the detector. However, if this mitigation is combined with exchangeable apertures, a new problem arises since the placement and / or orientation of the detector in relation to the X-ray source may need to be adapted for each respective aperture opening. This would entail mechanically moving and / or tilting the detector and the X-ray source in relation to each other. The size and mass of these objects discourages such solutions.

[0014] Thus, there is a desire to provide for exchangeable apertures within an X-ray source without contributing to secondary radiation.

[0015] The present invention is based on the recognition that secondary X-ray radiation can be prevented from reaching a detector used for imaging a sample, or even prevented from escaping the X-ray source altogether. Since a main source of secondary X-ray radiation is the electron beam limiting aperture, such secondary radiation can be prevented from reaching the detector by confining the propagation of secondary radiation inside an enclosure of the X-ray source such that it cannot reach the exit window. Typically, the exit window is constituted by the target itself, so preventing the secondary radiation from reaching the exit window can be accomplished by preventing the secondary radiation from reaching the target. By introducing a second aperture, acting as a limiting aperture for secondary radiation inside the X-ray source and through which the electron beam is passed before reaching the target, propagation of secondary radiation beyond the second aperture is confined and can thus be prevented from deteriorating or contaminating the primary X-ray output. Furthermore, by configuring the second aperture so that no secondary radiation is generated by interactions between the electron beam and the material surrounding the opening of the second aperture, the X-ray radiation emitted from the X-ray source may be comprised solely, or at least mainly, from radiation created by interactions between the electron beam and the target. This may be accomplished by making the second aperture opening larger than the lateral extent of the electron beam in the plane defined by the second aperture, such that the electron beam can pass freely therethrough without striking its edges.

[0016] Generally, the solution proposed herein involves deflecting the electron beam after is has passed the electron beam limiting aperture, i.e. the aperture that is used for limiting the angular distribution of the electron beam, and limiting an angular distribution of any secondary X-ray radiation generated at that aperture. As will be understood, the deflection of the electron beam (which may be accomplished electromagnetically or electrostatically) will not influence the propagation of X-ray radiation. A secondary radiation limiting element, e.g. an aperture, can therefore be positioned such that no secondary X-ray radiation reaches the exit window of the X- ray source, and thus cannot be detected with a detector used for detecting the primary X-ray radiation generated from interaction of the electron beam with the target. In a typical X-ray source, it is only through one or more intentional exit windows that the X-ray radiation can escape / exit the enclosure.

[0017] As used herein, the expressions "upstream" and "downstream" designate locations relative to a propagation direction of the electron beam. Hence, upstream means closer to the electron source, and downstream means further from the electron source.

[0018] In order to provide selectable electron beam apertures within an X-ray source without causing secondary radiation to contaminate the X-ray output, embodiments of the present invention comprise an aperture plate with a plurality of aperture openings. Selection of which aperture opening to use for limiting the electron beam size may be done mechanically, e.g. by rotating the aperture plate, or electrically, e.g. by deflecting the electron beam towards a desired aperture opening. An aperture selector is therefore provided, which is configured to make the selected one of the electron beam limiting apertures and the electron beam path coincide with one another. In general, the aperture openings in the aperture plate are distributed away from the center of the plate. To prevent secondary radiation from being emitted from the X-ray source, a second aperture is provided downstream (in the direction of the electron beam) of the aperture plate. The opening of the second aperture is structured and arranged so that there is no direct line of sight from the openings in the aperture plate to the exit window of the X-ray source. The opening in the second aperture is further structured and arranged so that the electron beam is not limited by this opening, thus no secondary radiation is generated there.

[0019] There is thus provided an X-ray source comprising an electron source for providing an electron beam; a target configured to generate X-ray radiation by interaction with the electron beam; an X-ray transparent exit window; a plurality of electron beam limiting apertures, each configured to limit an angular distribution of the electron beam; a secondary radiation limiting aperture; a first deflector arranged to deflect the electron beam from the secondary radiation limiting aperture towards the target; a second deflector arranged to deflect the electron beam from a selected one of the electron beam limiting apertures towards the secondary radiation limiting aperture; and a third deflector arranged to deflect the electron beam from the electron source towards the selected one of the electron beam limiting apertures; wherein each of the electron beam limiting apertures and the secondary radiation limiting aperture are arranged so that there is no direct line of sight for secondary X- ray radiation from the electron beam limiting apertures to the exit window. Thus, the electron beam has to be deflected at least once after passage of the electron beam limiting aperture in order to pass through the secondary radiation limiting aperture and reach the target / exit window. The electron beam limiting aperture, the secondary radiation limiting aperture, and the target / exit window are thus noncollinear. Thereby, the primary X-ray radiation output from the X-ray source will be substantially free from such secondary radiation.

[0020] Alternatively, instead of using the third deflector, the X-ray source can comprise an actuator arranged to bring the selected one of the electron beam limiting apertures into the electron beam path. It is also conceivable to combine the third deflector with such an actuator in implementations where a deflection of the electron beam towards the electron beam limiting aperture is desired, e.g. for fine- tuning the direction of the electron beam or for providing a particular electron beam path geometry. Generally, an aperture selector is provided for selecting the appropriate electron beam limiting aperture.

[0021] The plurality of electron beam limiting apertures may conveniently comprise aperture openings of different diameters. Further, the plurality of electron beam limiting apertures may be provided in an aperture plate, and a center of each of the electron beam limiting apertures may be located along a circle. Preferably, the electron beam limiting apertures are evenly distributed in an angular direction along the circle. Generally, the electron beam limiting apertures are preferably formed in a plate or screen that is substantially impenetrable for X-ray radiation, which means that any secondary X-ray radiation downstream of the electron beam limiting apertures emanate predominantly from regions adjacent to the aperture openings (or, more particularly, from regions adjacent to the selected one of the aperture openings). In some implementations, the first deflector may further be configured to deflect the electron beam towards a desired location on the target. In some implementations, the X-ray source comprises a separate scanning deflector configured to deflect the electron beam towards a desired location on the target, the scanning deflector being arranged downstream of the first deflector. Typically, the target comprises a substrate and a metal layer (i.e. a layer comprising at least one metallic element) configured to generate X-ray radiation upon impact of the electron beam. The substrate of the target may constitute an exit window for X-ray radiation to exit the X-ray source.

[0022] Brief of the

[0023] In the following detailed description, reference is made to the accompanying drawings, on which:

[0024] Fig. 1 schematically shows a first implementation of an X-ray source according to the principles disclosed herein;

[0025] Fig. 2 schematically shows an aperture plate comprising a plurality of electron beam limiting aperture openings;

[0026] Fig. 3 schematically shows a second implementation of an X-ray source according to the principles disclosed herein;

[0027] Fig. 4 schematically shows a third implementation of an X-ray source according to the principles disclosed herein; and Fig. 5 schematically shows a fourth implementation of an X-ray source according to the principles disclosed herein.

[0028] Detailed description

[0029] Fig 1 illustrates an X-ray source 100 according to the invention, comprising an electron beam generator 102 arranged for providing an electron beam 104. X-ray radiation 120 is generated from interactions between the electron beam 104 and a target 118. As shown, the target 118 (or more particularly the substrate thereof) may constitute an X-ray transparent exit window for X-ray radiation to exit the X-ray source. In other implementations, a separate X-ray transparent exit window is provided and the target (with its X-ray transparent substrate) is located upstream from the exit window. A beam deflector 106 is operative to deflect the electron beam towards an aperture plate 108. The aperture plate comprises a plurality of aperture openings each of which is configured to limit an angular distribution of the electron beam. The aperture openings in the aperture plate have different diameters, so that different angular distributions of the electron beam can be obtained by different selections of aperture openings. For illustrative purposes, two aperture openings 108a, 108b are shown. It will be understood, however, that more than two aperture openings may be provided in the aperture plate 108. The electron beam is deflected either to first aperture opening 108a along a first path 104a or to second aperture opening 108b along a second path 104b. After passing through the aperture plate 108, the electron beam is deflected by an aperture deflector 110 towards a second aperture 112 structured and arranged so that secondary radiation generated by interaction between the electron beam 104 and the aperture plate 108 cannot be emitted from the X-ray source. Typically, preventing the secondary X- ray radiation from being emitted from the X-ray source is accomplished by preventing the secondary X-ray radiation from reaching the target 118, which in this example constitutes the exit window. Downstream of the second aperture 112, a target deflector 114 and a focus lens 116 are arranged to deflect and focus the electron beam 104 towards the target 118. The focus lens 116 is provided between the target deflector 114 and the target 118 to focus the electron beam to a desired size on the target. By interactions between the electron beam and the target, X-ray radiation 120 is generated and emitted from the X-ray source. The target deflector 114 may also be used as a scanning deflector to move the electron beam spot on the target. As an alternative, a separate scanning deflector (not shown) may be provided.

[0030] In the embodiment shown in Fig. 1, secondary X-ray radiation that is generated at the aperture plate 108 cannot reach the target 118, and thereby cannot exit from the X-ray source. Such secondary X-ray radiation will propagate radially from the selected aperture in the aperture plate 108 and can thus only reach parts of the X-ray source downstream from the second aperture 112 that have an open line of sight to the selected aperture in the aperture plate 108 passing through the second aperture 112. Emission of such secondary radiation through the target 118 is thereby prevented. As will be understood, the X-ray source typically has an enclosure that is opaque to (or at least significantly reduces) X-ray radiation, such that X-ray radiation can only escape through one or more X-ray windows. Since the second aperture 112 has the purpose of limiting the secondary X-ray radiation, and not to constrain the electron beam, the diameter of the second aperture can be considerably larger than the electron beam limiting apertures.

[0031] Fig 2 illustrates an aperture plate 200 according to the invention. The aperture plate comprises, in this example, three aperture openings 202, 204, 206. The aperture openings are distributed around a center point 208 of the aperture plate along a circle indicated at 210. The aperture openings are provided sufficiently spaced apart to prevent crosstalk, i.e. electrons in the electron beam should pass through only the selected aperture opening towards which the electron beam is directed. As shown, the aperture openings may have different diameters so that different limitations of the angular distribution of the electron beam can be selected. For example, aperture opening 202 has a smaller diameter than aperture opening 204, which in turn has a smaller diameter than aperture opening 206. In embodiments, the aperture plate is preferably rotated about its center point 208 to bring a selected one of the aperture openings into the electron beam path. Rotation of the aperture plate can be effected e.g. using a motor / actuator. Conveniently, each of the aperture openings 202, 204, 206 are thus centered along the circle 210.

[0032] Fig 3 illustrates an alternative embodiment in which aperture selection is performed mechanically by rotating the aperture plate. X-ray source 300 comprises an electron beam generator 302 arranged for generating an electron beam 304. X- ray radiation 320 is generated from interactions between the electron beam 304 and a target 318. An aperture plate 308 is structured and arranged so that a selected one of a plurality of aperture openings 308a, 308b can be positioned, by means of a motor or actuator 303, in the electron beam path and thereby limit the angular distribution of the electron beam. An aperture deflector 310 is arranged to deflect the electron beam towards a second aperture 312 structured and arranged so that secondary X-ray radiation generated by interaction between the electron beam and the aperture plate 308 cannot be emitted from the X-ray source. A target deflector 314 is arranged downstream of the second aperture 312 to deflect the electron beam towards the target 318. Between the target deflector 314 and the target 318, there is provided a focus lens 316 to focus the electron beam to a desired size on the target 318. By interactions between the electron beam 304 and the target 318, X-ray radiation 320 is generated and emitted from the X-ray source illuminating a sample 322 supported by sample stage 324. A detector 326 is provided for imaging of the sample 322.

[0033] Fig 4 illustrates an alternative embodiment in which aperture selection is performed mechanically by rotating the aperture plate. The X-ray source 400 comprises an electron beam generator 402 arranged for generating an electron beam 404. X-ray radiation 420 is generated from interactions between the electron beam 404 and the target 418. An aperture plate 408 is structured and arranged so that a selected one of a plurality of aperture openings 408a, 408b can be positioned, by means of a motor or actuator 403, in the electron beam path and thereby limit the angular distribution of the electron beam. A second aperture 412 is arranged in line with the selected aperture opening so that the electron beam may pass through without interacting with the material in second aperture 412 while secondary radiation 427 generated by interaction between the electron beam 404 and the aperture plate 408 is prevented from reaching the target / exit window 418. As shown schematically in the figure, secondary radiation 427 emitted from the edge of the selected aperture opening 408b that passes through the second aperture 412 will be prevented from exiting the X-ray source by enclosure 428. A target deflector 414 is arranged downstream of the second aperture 412 to deflect the electron beam towards the target 418. Between the target deflector 414 and the target 418, there is provided a focus lens 416 to focus the electron beam to a desired size on the target 418. By interactions between the electron beam 404 and the target 418, X-ray radiation 420 is generated and emitted from the X-ray source.

[0034] Fig 5 illustrates an alternative embodiment in which aperture selection is performed by mechanically rotating the aperture plate. X-ray source 500 comprises an electron beam generator 502 arranged for generating an electron beam 504. X- ray radiation 520 is generated from interactions between the electron beam 504 and the target 518. An aperture plate 508 is structured and arranged so that a selected one of a plurality of aperture openings 508a, 508b can be positioned, by means of a motor or actuator 503, in the electron beam path and thereby limit the angular distribution of the electron beam. A second aperture 512 is arranged in line with the selected aperture opening so that the electron beam may pass through without interacting with the material in second aperture 512 while secondary radiation 527 generated by interaction between the electron beam 504 and the aperture plate 508 is prevented from reaching the target / exit window 518. As shown schematically in the figure, secondary radiation 527 emitted from the edge of the selected aperture opening 508b that passes through the second aperture 512 will be prevented from exiting the X-ray source by enclosure 528. A target deflector 514 is arranged downstream of the second aperture 512 to deflect the electron beam towards the target 518. Between the target deflector 514 and the target 518, there is provided a focus lens 516 to focus the electron beam to a desired size on the target 518. By interactions between the electron beam 504 and the target 518, X-ray radiation 520 is generated and emitted from the X-ray source illuminating a sample 522 supported by sample stage 524. A detector 526 is provided for imaging of the sample 522.

[0035] The embodiments shown in Fig. 4 and Fig. 5 comprise one less deflector than the embodiment of Fig 3. Instead, the electron source and the target are arranged at an angle relative to each other.

[0036] It should be noted that a deflector and an aperture may be arranged in several ways to achieve the intended functionality according to the invention. For example, apertures 112 and 312 arranged upstream of deflectors 114 and 314 respectively in Figs. 1 and 3 may instead be arranged downstream of the respective deflector. In a further alternative, the aperture and the deflector may be integrated so that deflection is performed in a plane defined by the aperture. For illustrative purposes, a sample, a sample stage, and a detector are not shown in Fig. 1 or Fig. 4, although it will be understood that such are normally present for imaging applications.

[0037] Secondary radiation mitigation in electron-impact X-ray sources has been disclosed. Embodiments of the present invention combine the possibility to select a beam-limiting aperture opening for limiting the angular distribution of the electron beam with a highly effective way of preventing secondary X-ray radiation, particularly secondary radiation generated by interaction between the electron beam and the beam-limiting aperture, from exiting the X-ray source and thereby contaminate the X-ray output by the use of a secondary radiation limiting aperture downstream from the electron beam limiting apertures.

[0038] Arrangements, sources, and methods according to the invention may be used for different types of X-ray imaging such as X-ray microscopy, radiography, fluoroscopy, laminography, or CT scanning.

Claims

CLAIMS1. A transmission-type X-ray source, comprising an electron source for providing an electron beam; a target configured to generate X-ray radiation by interaction with the electron beam; an X-ray transparent exit window; a plurality of electron beam limiting apertures, each configured to limit an angular distribution of the electron beam; a secondary radiation limiting aperture; a first deflector arranged to deflect the electron beam from the secondary radiation limiting aperture towards the target; and an aperture selector configured to make a selected one of the electron beam limiting apertures and the electron beam path coincide; wherein each of the electron beam limiting apertures and the secondary radiation limiting aperture are arranged so that there is no direct line of sight for secondary X-ray radiation from the selected one of the electron beam limiting apertures to the exit window.

2. The X-ray source of claim 1, further comprising a second deflector arranged to deflect the electron beam from the selected one of the electron beam limiting apertures towards the secondary radiation limiting aperture; and wherein the aperture selector comprises a third deflector arranged to deflect the electron beam from the electron source towards the selected one of the electron beam limiting apertures.

3. The X-ray source of claim 1, wherein the aperture selector comprises an actuator arranged to bring the selected one of the electron beam limiting apertures into the electron beam path.

4. The X-ray source of claim 3, comprising a second deflector arranged to deflect the electron beam from the selected one of the electron beam limiting apertures towards the secondary radiation limiting aperture.

5. The X-ray source of any one of the preceding claims, wherein the plurality of electron beam limiting apertures comprises aperture openings of different diameters.

6. The X-ray source of any one of the preceding claims, wherein the plurality of electron beam limiting apertures are provided in an aperture plate, and wherein a center of each of the electron beam limiting apertures is located along a circle.

7. The X-ray source of claim 6, wherein the electron beam limiting apertures are evenly distributed in an angular direction along the circle.

8. The X-ray source of any one of claims 1-7, further comprising a scanning deflector configured to deflect the electron beam towards a location on the target, the scanning deflector being arranged downstream of the first deflector.

9. The X-ray source of any one of the preceding claims, wherein the target comprises a substrate and a metal layer configured to generate X-ray radiation upon impact of the electron beam.

10. The X-ray source of claim 9, wherein the substrate of the target constitutes the exit window for X-ray radiation to exit the X-ray source.

11. The X-ray source of claim 9 or 10, wherein the substrate comprises diamond and the metal layer comprises tungsten.

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